Rhenium (Re) is a rare, greyish-white, heavy, poly-valent transition metal, of atomic number 75 and atomic weight 186.2. In its chemical properties, it resembles manganese. It is extremely dense. It has a very high melting point (3186° C.); a property which leads to its use in high temperature resistant alloys, especially nickel-based superalloys used in jet aircraft engine parts such as turbine blades, and in blades for gas turbine generators. Rhenium confers both high temperature resistance and corrosion resistance on the alloys. A typical such alloy contains 2 to 6% by weight rhenium, along with a major proportion (50 to 60%) nickel and minor amounts of one or more of cobalt, chromium, aluminum, molybdenum, tantalum and tungsten (e.g. 2 to 10% of each).
Rhenium is also used, to a lesser extent, in catalysts for making lead-free, high octane gasoline, typically in conjunction with platinum.
There is a wide range of oxidation states of rhenium, the widest range of any known element. The known oxidation states are −3, −1, 0, +1, +2, +3, +4, +5, +6 and +7, with oxidation states +7, +6, +4, +2 and −1 being the most common. The major commercial source of rhenium is as a minor constituent (up to 0.2%) in the mineral molybdenite. It is extracted from molybdenite roaster-flue gas, by leaching, in the form of its water soluble oxides.
Rhenium is extremely expensive—among the ten most expensive metals known. At times, its price exceeds $10,000 per kilogram. As a result, recovery of the rhenium content of used alloys, e.g. from worn-out aircraft turbines and from other rhenium-containing compounds, is desirable.